Wide band antenna with integral reflector



Nov. 1, 1960 sxRo OKAMURA 2,958,868

WIDE BAND ANTENNA WITH INTEGRAL REFLECTOR Filed July 2o, 1955 2 Sheets-Sheet 1 Q 0 r A 00.200 300 40a Jaa 600 700,140 200. FfaafA/fy l 1N VEN TOR. 5/,90 0mm/,M

A f/mmw 197 70K/Vf' Y Nov. 1, 1960 SIR@ OKAMURA 2,958,868

WIDE BAND ANTENNA WITH INTEGRAL REFLECTOR Filed July 20, 1955 2 Sheets-Sheet 2 1N VEN TOR. BY 5MP@ //Mz//P United States Patent WIDE BAND ANTENNA WTH INTEGRAL REFLECTOR Siro Okamura, Tokyo, Japan- Filed July 20, 1955, Ser. No. 523,319

Claims priority, application Japan Aug. 3, 1954 13 Claims. (Cl. 343-795) This invention relates to wide frequency band antennas, and more particularly, though not exclusively, to antennas useful at very high frequencies (VHF) and at ultra high frequencies (UHF).

In VHF and UHF television systems, for example, antennas useful over exceedingly wide frequency bands are in demand. According to standards in the United States of America, VHF television utilizes frequencies between 54 and 216 megacycles, and UHF television utilizies frequencies between 470 and 940 megacycles. There are several types of wideband antennas which have been developed and put into practice. These antennas are relatively costly to manufacture, have smaller bandwidths than are desired, and may have an impedance which does not match the impedance of standard parallel wire feeders or transmission lines. These disadvantages appear singly or in combination. The gains of these known antennas are generally zero db over the standard doublet or dipole.

It is an object of the present invention to provide an improved wideband antenna which is relatively simple and easy to construct.

It is another object to provide an improved folded dipole antenna which has a substantially constant impedance over a wide frequency range and which is adapted for direct connection to a parallel wire feeder.

It is a further object to provide an improved unipole antenna which has a substantially constant impedance over a wide frequency range and which is adapted for direct connection to a coaxial line feeder.

It is a still further object to provide an improved antenna arrangement having an integral antenna element and reflector.

In one aspect the invention comprises a folded dipole antenna constructed of a sheet of metal or metallic screen or mesh having a generally diamond shape when at and having a centrally located aperture. Two opposite corners of the sheet are folded back towards each other in smooth curves to form the front side of the antenna, and the corners are passed through the aperture. A parallel wire feeder is connected from the back side of the antenna 'to the two corners.

In another aspect the invention comprises a folded unipole antenna constructed of a sheet of metal or metallic screen or mesh having a generally triangular shape when flat and having an aperture positioned near one side of the triangular sheet. The corner of the sheet opposite from the aperture is folded back in a smooth curve to form the front side of the antenna, and the corner is passed through the aperture. A coaxial line feeder is connected to the back side of the antenna with the inner conductor connected to the corner and the outer conductor connected to the edges of the aperture.

These and other objects and aspects of the invention will be apparent to those skilled in the art from the following more detailed description taken in conjunction with the appended drawings, wherein:

Figures la through ld illustrate an antenna fashioned ICC from a metal sheet or wire mesh according to the teachings of this invention;

Figures 2a and 2b are charts showing the impedance vs. frequency characteristics of two antennas according to Figure l and having specied dimensions;

Figures 3a and 3b are charts showing the radiation patterns at two different frequencies of the antenna having the impedance characteristics shown in Figure 2a;

Figure 4 illustrates an antenna according to this invention made with a relatively small number of metallic wires spaced to provide a current sheet;

Figure 5 shows a folded unipole antenna according to the invention and adapted for direct connection to a coaxial line feeder; and

Figure 6 illustrates an antenna according to Figure l in combination with a parabolic reflector.

Figure 7 is a perspective view of the antenna of Figure 1.

Figures la through 1d illustrate a folded dipole antenna according to the invention. Figure la shows the shape of a sheet 6 of metallic plate or wire mesh as it appears when flat prior to forming into the antenna shown in elevation, plan and side views in Figures 1b, 1c and 1d, respectively. The term sheet, as used herein, is intended to cover acurrent sheet which may be a plate or a plurality of relatively closely spaced wires. lt will be noted that the conductive sheet 6 has a generally diamond shape with two more widely spaced opposite corners 1 and 2, and two more closely spaced opposite corners 4 and 5. The sides of the diamond shaped sheet preferably follow an inwardly extending curve so that the width of the sheet from either of the corners 1 or 2 towards the center increases at a greater than linear rate, viz., at an exponential rate. An aperture 7 in the plate or sheet 6 is preferably centered in the sheet, but it may be off center if desired.

Referring to Figures lb through ld and 7, the antenna is constructed from the flat sheet shown in Figure 1a by folding back the more widely spaced corners 1 and 2 towards each other in smooth curves, the corners 1 and 2 being passed through the central aperture 7 in the sheet. The corners 1 and 2 extend through the aperture 7 to the back side of the antenna, the other side being the front side. The corners 1 and 2 are held in insulated relationship with each other and with the edges of the aperture 7 by means of an insulating member 3 which may be of any suitable insulating material such as plastic. The corners 1 and 2 are adapted for direct connection to the two respective conductors of a parallel wire feeder or transmission line 8.

Figure 2a shows the impedance vs. frequency characteristics of an antenna according to Figure 1 and having the following dimensions:

a=17.3 inches b=8.28 inches c: 19.7 inches d: 1.18 inches The curve labeled R represents resistance and the curve labeled X represents reactance. It will be noted that the antenna has an impedance in the order of 200y to 300 ohms over a very broad frequency range. The impedance of the antenna provides a good match with a standard 300 ohm parallel wire transmission line. Because of the flexibility of the sheet, changes in the impedance of the antenna can be accomplished by varying the shape ofthe conductive sheet 6 shown in Figure la, and by Varying the curvatures in which the sheet is formed. The characteristics may also be altered by bending the corners 4 and 5 forwardly or backwardly.

The chart of Figure 2a shows that the antenna of this invent-ion has a substantially constant impedance over a broad frequency range. This desirable characteristic is due to the tapered shape of the conductive sheet from the feed ends o-r corners 1 and 2 towards the central portion of the antenna. Therefore, the antenna of this invention has the desirable wideband characteristics found in an exponential line damped with the radiation resistance and in a bow-tie antenna. The antenna of this invention is superior to bow-tie antennas which generally have a low input impedance requiring a matching circuit, such as a quarter-wave line, between the transmission line and the antenna feed points. The use of a matching circuit tends to impair the frequency response characteristic which would otherwise be obtained.

From the data on the chart it can be shown that for a voltage standing wave ratio less than 2, the impedance vs. Vfrequency response curve is substantially ilat over a frequency bandwidth of three times the lowest frequency. For a voltage standing wave ratio of less than 1.4, the useful bandwidth is twice the lowest frequency.

Figure 2b is a chart similar to that of Figure 2a but showing the characteristics of an antenna according to Figure 1 and having dimensions as follows:

a: 10.4 inches b=6.1 inches c=13.8 inches d=l.l8 inches The dimensions given in connection with the charts of Figures 2a and 2b are merely illustrative of antennas constructed according to the invention.

Figu-res 3a and 3b show the radiation patterns of an antenna having the dimensions recited in connection with the chart of Figure 2a. Figure 3a shows the pattern at a frequency of about 350` megacycles and Figure 3b shows the pattern at a frequency of about 600 megacycles. The gain of the antenna was measured to be about three db over a standard doublet or dipole at a frequency or" 450 megacycles. At other frequencies the gain drops slightly.

The unidirectional properties of the antenna of this invention result from the construction which provides an antenna element and also a reflector mechanically integral with the antenna element. The central area of the conductive sheet constitutes the reilector, and the folded back portions adjacent the corners 1 and 2. constitute the antenna element. rPhe central or wide portion of the antenna constitutes a reliector having wider bandwidth ycharacteristics than are normally obtained with reflectors constructed of conductive bars.

From the fo-regoing it is apparent that the antenna of this invention is characterized in having (l) an input impedance providing a good impedance match with a standard two-Conductor parallel wire feeder, (2) an input impedance which is substantially constant over a very broad frequency band, and (3) unidirectional properties due to the cooperation of an antenna element and a wideband reflector.

Figure 4 illustrates an invention generally similar to that of Figure l but constructed of a plurality of individual metallic wires forming a conductive sheet. The use of wire mesh for the antenna of Figure 1 and the use of wires in the antenna of Figure 4 reduces the wind pressure on the antenna in outdoor use without appreciably altering the electrical properties of the antenna if reasonable care is taken in the construction.

Figure 5 shows an antenna according to this invention in the form of a folded unipole. A thin sheet of conductive plate or mesh from which the antenna is constructed has a generally triangular shape when at and has an aperture located near one side 11 of the triangle. The corner 12 of the triangular sheet opposite the edge 11 is folded back in a smooth curve and passed through the aperture 10. A coaxial feed line 13 on the backside o-f the antenna is connected with the outer conductor in contact with the edge of the aperture 1Q and with the inner conductor in contact with the end of the corner 12. An insulating washer in the coaxial line or in the aperture lmaintains the corner 12 in insulated spaced relationship with the edges of the aperture 10. It will be noted that the folded unipole shown on Figure 5 is essentially half of the folded dipole of Figure 1. The unipole of Figure 5 is especially well adapted for use with a coaxial line feeder.

Figure 6 shows a folded dipole 15 constructed in the manner shown in Figure l and having a parallel wire transmission line 16 connected thereto. The antenna 15 is positioned at the focal point lof a parabolic reflector 17. The antenna 15 in combination with the reflector 17 provides a highly directional beam pattern which is especially useful in microwave communication systems. As with all antennas, it may be used for transmitting or receiving radio frequency energy. The antenna of Figure 6 is also useful for microwave field intensity measurements, where microstrips may be used, because in such apparatus the frequency changes widely.

What is claimed is:

l. A wideband antenna comprising a sheet of conductive material having at least a generally triangular portion when dat, one corner of said sheet being folded around in a smooth curve to the vicinity of an imaginary straight line extending between the other two corners of said triangular portion, and means to connect a feed conductor to said one corner.

2. A wideband antenna comprising a sheet of conductive material having at least a generally triangular portion when at, one corner of said sheet being folded around in a smooth curve to the vicinity of an imaginary straight line extending between the other two corners of said triangular portion, a coaxial feeder, said feeder having an inner conductor connected to said one corner and an outer co-nductor connected to said sheet in the vicinity of said straight line.

3. A wideband antenna comprising a sheet of conductive material having at least a generally triangular portion when at and having an aperture therein in the vicinity of a line between two corners of said triangular portion, the third corner of said triangular port-ion being folded around in a smooth curve to said aperture, and means to connect a feed `conductor to said third corner by a path extending through said aperture.

4. A wideband antenna comprising, a sheet of conductive material having a generally diamond shape when at and having an aperture therein, two opposite corners of said sheet being folded around to said aperture, and means to connect a transmission line to said corners by paths extending through said aperture.

5. A wideband antenna comprising, a sheet of conductive material having a generally diamond shaped outline when flat and having a central aperture therein, two opposite corners of said sheet being formed towards each other and through said aperture, and a transmission line having two conductors connected to respective ones of said corners.

6. A wideband antenna as defined in claim 5, and in addition, an insulating member at said aperture to maintain said opposite ycorners in insulated relationship with each other and with the edges of said aperture.

7. A wideband antenna as defined in claim 5 wherein said conductive sheet is a continuous metallic sheet.

8. A wideband antenna as defined in claim 5 wherein said conductive sheet is a mesh of conductive wires.

9. A wideband antenna as delined in claim 5 wherein said conductive sheet is constituted by a plurality of wires, one end of each of said wires being connected to one of said ltwo corners and the other end of each of said wires being connected to the other of said two corners.

10. A wideband antenna comprising, a sheet of conductive material having a generally diamond shaped outline when fiat and having an aperture therein, the edges of said sheet being inwardly curved to provide an exponential taper, two opposite :corners of said sheet being formed towards each other in smooth curves to said aperture, an insulating member at said aperture supporting said two corners in said aperture in insulated relationship with each other and with the edges of said aperture, and a parallel conductor transmission line having conductors `connected respectively to said two corners.

ll. A wide band antenna comprising a sheet of conductive material having a generally triangular shape when flat, and having an aperture in said sheet adjacent one edge thereof, the corner of sai-d sheet opposite said one edge being folded around in a smooth curve to said aperture, a coaxial conductor, and means to connect the inner conductor of said coaxial conductor to said one corner by a path extending thru said aperture, and means to connect the outer conductor of said coaxial conductor to said sheet adjacent said aperture.

12. An antenna comprising a flexible thin plate-like conductive element comprising an initially generally triangular shaped portion having a base and elongated sides extending therefrom to an apex, the base of the conductive element having an opening therethrough near the edge thereof, the element being curved upon itself such that the apex extends through said opening, the folding shape of the conductive element being Variable by flexibility of said element, whereby a modification of the radiation characteristic may be eifected.

13. An antenna for electromagnetic waves comprising an initially llat conductive element having a relatively broad base and a portion extending from the base having sides converging substantially to a sharp point, said base having an opening through the element, said element bein-g curved upon itself with the point of the element extending through the opening in the base, whereby the base operates substantially as a wave reflector.

References Cited in the le of this patent UNITED STATES PATENTS Poirier Oct. 2, 1928 Garnet Feb. 13, 1945 Albright June 16, 1953 

